The intent of this proposal is to further our understanding of the antigens of T.gondii which induce host immunity. Identification of these antigens will allow for improved diagnosis and potential immunotherapy in those afflicted with AIDS. It has been estimated that as many as 1/3 of all AIDS patients will develop toxoplasma meningoencephalitis during the course of their illness. Our first specific aim is to improve serodiagnosis by measuring specific antibody responses to either native or recombinant proteins in an experimental model that mimics AIDS: The CD4-depleted mouse. We also plan to develop tests which detect circulating T.gondii antigens such as P30, P22 and other excreted/secreted antigens in the body fluids of an immunosuppressed model. Antibodies reactive to these antigens would provide an alternative to current, less reliable, serodiagnostic methods used in AIDS patients. Another specific aim is to evaluate the ability of both native and recombinant P3- to induce protective immunity in susceptible hosts. P30 induces parasiticidal antibody as well as cytotoxic T-cells in vivo, and it has been successfully expressed as a recombinant fusion protein in bacteria. We plan to determine the ability of various immunodominant T-cell epitopes of P30 to act as a subunit vaccine in the experimental immunosuppressed model. We also plan to evaluate another major parasite antigen,l P22, that is recognized by the sera from AIDS patients. Polyclonal antisera and anti-P22 mAbs will be used to isolate and characterize a fusion protein expressed in our lambda gt11 cDNA library. The fusion protein will be assayed for its potential to improve serodiagnosis in AIDS patients. The final specific aim is to complete the isolation and characterization of the specific parasite antigen(s) that stimulate human cytotoxic T-cells. Previous studies in our laboratory indicate that unique T-cell antigens can be identified and purified that induce a population of CD8+, alpha, beta heterodimer positive cells that are directly cytotoxic to extracellular parasites. We plan to characterize this antigen(s) and isolate corresponding fusion proteins from a cDNA library. These will be tested for the ability to induce immunity against T.gondii infection in an experimental host.